There is a great need in the food industry for a sweet, zero calorie, non-toxic sugar. An ideal example would be a nonmetabolizable fructose. D-Fructose, the sweetest natural sugar, provides excellent organoleptic acceptance and mouth feel. The United States uses over 23 billion pounds of sucrose per year and some of it is hydrolyzed to its constituent sugars namely one unit of D-glucose and one unit of D-fructose. The mixture is commercially called invert sugar. Since D-fructose is 1.7 times as sweet as sucrose, and D-glucose 0.7 times as sweet as sucrose, the total sweetness of invert sugar is higher than that of sucrose. With the advent of enzyme engineering, it has become possible to convert D-glucose to an equilibrium mixture of D-glucose and D-fructose in a proportion of 54% to 46% respectively, and hence, produce the sweetness of invert sugar from low cost D-glucose, a product of the corn wet milling industry. As a consequence, the use of fructose in the food industry has increased extensively.
With all of the desirable advances in producing the sweet sugar D-fructose, there has been no reduction in caloric value except for the lesser amount of D-fructose required to give the same sweetness as sucrose. Both D-glucose and D-fructose are completely and equally metabolized in humans to produce equivalent caloric energy.
With the health need to take in less calories, some of the normal metabolic sugars should be replaced with other acceptable nonmetabolizable and non-toxic sweeteners. This has been done with the development of synthetic sweeteners such as saccharin and aspartame. However, a much more desirable sweetener is L-fructose which has the same sweetness as natural D-fructose. It has no toxicity and seems not to be metabolized by humans, thus yielding no caloric value. L-Fructose is the mirror image of D-fructose. It has the same taste and most of the other properties of D-fructose but is not a substrate for the metabolic enzymes of the human system.
Examples of sweetened substrates which may be prepared with L-fructose can be found in U.S. Pat. No. 4,262,032 to Levin which is expressly incorporated herein. This patent describes and claims the use of L-fructose and other L-hexose monosaccharides as sweetening agents.
L-Fructose, which has not been found in nature, was synthesized for the first time by Fischer. See E. Fischer, Ber., 23 (1890) 370-394. DL-Glucose phenylosazone was prepared from .alpha.-acrose and hydrolyzed to the glycosulose, which was reduced to DL-fructose. L-Fructose was isolated from the mixture. L-Fructose was later synthesized by Wolfrom and Thompson from L-arabinonic acid in five steps. See M. L. Wolfrom and A. Thompson, Methods of Carbohydr. Chem., 1 (1962) 118-120.
Recently, L-fructose was prepared by aldol condensation. See S. Morgenlie, Carbohydr. Res., 107 (1982) 137-141. DL-Glyceraldehyde condensed with 1,3-dihydroxy-2-propanone catalyzed by Dowex 1 (OH.sup.-) resin to give a hexulose mixture. Crystallization of the mixture from methanol yielded 54% of DL-fructose. Treatment of the DL-fructose mixture with baker's yeast gave a product from which 62% of 2,3:4,5-di-O-isopropylidene-.beta.-L-fructopyranose was isolated after acetonation. When L-glyceraldehyde was used as starting material, 2,3:4,5-di-O-isopropylidene-.beta.-L-fructopyranose was isolated in 60-65% yield after acetonation of the hexulose mixture.
L-Fructose has been prepared enzymatically from L-mannose by an isomerase present in cell-free extracts of Aerobacter aerogenes grown on L-mannose to give L-fructose in 28-32% yield. See J. W. Mayo and R. J. Anderson, Carbohydr. Res., 8 (1968) 344-347.
Both chemical and enzymic procedures either are too elaborate or require relatively expensive starting material. To commercialize L-fructose as a sweetener requires a low cost process. A simple method for the synthesis of L-fructose which starts with L-sorbose, a relatively inexpensive industrial chemical would be desirable. The only structural difference between L-sorbose and L-fructose is the reverse configuration of the hydroxyl groups at C-3 and C-4.